222 MOUNTING OF OBJECTS. 



not be remitted, until the varnish is dry enough to hold down 

 the cover by itself. For mounting delicate objects, the thinner 

 slides should be selected (§ 116) ; and for very difficult test-objects, 

 it is advantageous to employ thin glass below as well as above 

 the specimens, for the sake of diminishing the aberration which 

 the illuminating pencil sustains in its passage to the object, and 

 for allowing the achromatic condenser to approach the object as 

 closely as possible. For this purpose, the simplest method is to 

 take a slip of wood, of the ordinary size of the glass slide (3 in. 

 by 1 in.), with a central aperture of from 3 to 5-8ths of an inch ; 

 to cover this aperture with a " square" or " round" of thin glass 

 of sufficient size to project considerably beyond it; to lay the 

 object upon this glass, and to protect it with a cover of rather 

 smaller size, which should be fastened down all round by var- 

 nish, to prevent the entrance of moisture ; and finally to secure 

 both glasses to the wooden slide, by gumming down over them 

 a piece of paper, with a perforation of the same size as that of 

 the slide itself 



123. For dry-mounting opaque objects, the method adopted 

 must vary with the inode in which the object is to be illuminated; 

 since, if a side-condenser or reflector is to be employed, the whole 

 slide may be opaque ; whereas if the Lieberklihn is preferred, 

 the object should be placed on a disk of appropriate size (§ 65), 

 supported in such a mode as to admit the light all around it. 

 For the former purpose, the Author has devised the following 

 simple method, which he has found to affiDrd peculiar con- 

 veniences. Let there be provided a cedar slide of the kind just 

 described, a piece of card of the same dimensions, and a piece of 

 t^eac^-black paper, rather larger than the aperture of the slide, if a 

 dark mounting be desired, which is preferable for most objects ; 

 this piece of paper is to be gummed to the middle of the card, 

 and then, some stiff gum having been previously spread over one 

 side of the slide (care being taken that there is no superfluity of 

 it immediately around the aperture), this is to be laid down upon 

 the card, and subjected to pressure." An extremely neat " cell" 

 will thus be formed for the reception of the object, the depth of 

 which will be determined by the thickness of the slide, and the 

 diameter by the size of the perforation ; and it will be found con- 

 venient to use slides of various thicknesses and having apertures 

 of different sizes. The cell should always be deep enough for its 

 wall to rise above the object ; but, on the other hand, if it be too 

 deep, its wall will interfere with the oblique incidence of the 

 light upon any object that may be near its periphery. The ob- 

 ject, if flat or small, maybe attached by ordinary gum -mucilage ; 

 if, however, it be large, and the part of it to be attached have an 

 irregular surface, it is desirable to afford a "bed" to this by gum 



' It will be found a very convenient plan, to prepare a large number of such slides at 

 once ; and this may be done in a marvellously short time, if the slips of card have been 

 previously cut to the exact size in a bookbinder's press. 



